The present invention relates to tensioning devices, and more particularly to devices for tensioning a rod before the rod is tightened in position.
Pre-stressed threaded rods used to tighten two mechanical structures together are known. These threaded rods may be used particularly in wind turbines or nuclear reactors in order to maintain firmly two mechanical structures together. The tensioning of the rods can be done by applying a tightening torque to nuts resting axially on the surfaces of the structures to be tightened. This tightening method does not allow controlling precisely the axial force transmitted to the rod, because of friction between the nut in contact with the surface of the part to be tightened and between the nut and the rod. Moreover, this tightening method introduces torsional residual stresses into the threaded rod, which results in the necessity of using oversize threaded rods. In addition, such a tightening generally leads to a significant risk of damage to the threads of the nut and of the rod cooperating together during tightening. The surface against which the nut is resting during its rotation at the time of tightening may also be damaged.
In order to avoid these disadvantages, a threaded rod may be axially tightened before tightening a nut on the surface of a structure to be fastened. The nut blocks the rod when the axial force transmitted to the rod is released. The rod is thus pre-stressed only along its axis. An actuator can be used in order to transmit an axial force to the rod. Thus, after the nut is positioned on the threaded portion of the rod to be tensioned, the actuator is positioned so as to surround the threaded portion and the nut. Large mechanical assemblies must often be maintained by several threaded rods. The implementation of such tensioning devices requires, especially when the number of threaded rods is important, multiple screwing and unscrewing of the ties which makes the assembly operations long and difficult to realize.
French patent application FR 2841304 (SKF) discloses a rod tensioning device comprising an actuator, a tie-rod transmitting an axial force from the actuator to the threaded rod. The tie-rod includes two jaws, which can move radially between a closed position, in which the tie-rod is engaged with the threaded portion of the rod, and an open position, in which the jaws are disengaged from the threaded rod. The tensioning device includes a locking element sliding inside the jaws in order to close or open the jaws. The locking element includes radial flanges in order to open the jaws. When the locking element is moved downward towards the threaded rod, the jaws are disengaged from the threaded rod and when the locking element is moved upward, the jaws are engaged with the threaded rod. The dimensions of such a tensioning device make the tensioning of several threaded rods on a small device difficult and the tensioning device is also uneasy to manipulate. Moreover, the jaws are not guided very precisely.